The present invention concerns an apparatus and a method for detecting a rotation, especially an apparatus and a method for detecting a rotation on the basis of bulk acoustic waves.
Conventional approaches for detecting a rotation about an axis use gyroscopes, e.g., MEMS gyroscopes (MEMS=micro-electromechanical system). Conventional MEMS gyroscopes are complicated vibrating micro-structures manufactured by surface-micromachining processes. Gyroscopes are based on measuring the Coriolis force. The Coriolis force is a small force perpendicular to both the velocity vector of a mass and the rotation axis. In a vibratory gyroscope the Coriolis force is proportional to a rotation (yaw) rate, the vibration frequency and the mass of the vibrating mass.
In general, the Coriolis force is detected capacitively on the basis of a displacement of sensing electrodes in a direction perpendicular to the direction of the driven motion. Major problems occur in gyroscopes due to the so-called quadrature errors; these errors are related to non-ideal drive motion or driving force coupling energy into the sense or detection direction. As the Coriolis force results in a displacement, which is, in general, by a factor of 10,000 smaller than the driven motion, the construction of MEMS gyroscopes is challenging and the electronic circuitry to operate the device is complex. Conventional MEMS gyroscopes are too expensive for true volume applications while they do not have the necessitated degree of accuracy needed in specific applications, e.g., in navigation systems.
An improvement of the sensitivity of gyroscopes can be obtained by maximizing a sensing or detection signal generated due to the Coriolis force. Maximizing the detection signal is possible by (a) increasing the mass in the gyroscope, (b) increasing the vibration frequency, (c) increasing the vibration amplitude, and/or (d) improving the displacement sensing.
The first approach (a) is disadvantageous in that also the size of the overall device needs to be increased. Further, manufacturing such a device becomes difficult. The second approach (b) is not possible with MEMS gyroscopes since these devices work best at 10 kHz. At higher frequencies the vibration amplitude becomes smaller which, however, would be in contradiction to (c). The third approach (c) is rather limited with MEMS gyroscopes as the suspension structures, like springs, and the driving force generating structures, like comb drives, do not allow a displacement exceeding ±5 μm. The fourth approach (d) is limited by the above mentioned quadrature error and the noise limit of the amplifiers used in the detection circuitry.
Conventional gyroscopes use a combination of (a), (c) and (d) to increase the detection limit.